Ingenol-3-angelate has been described previously as being useful in treating a number of disorders, in particular actinic keratosis. The compound can be isolated from a natural source, and is present in relatively small amount in plants of the Euphorbia plant family. The related compound, ingenol, however, can be isolated from natural sources in larger quantities. The present invention thus provides a process for preparing ingenol-3-angelate starting from ingenol.